Long-life satellites

日期:2017-04-16 07:58:01 作者:舜彬 阅读:

By Barry Fox in Baikonur A MOTOR that uses jets of ionised gas to keep spacecraft in their allotted place in the sky could revolutionise the economics of satellite TV and communications. Today’s satellites have a working life of around 10 years, after which they have to be replaced. The new ion propulsion system, which went into orbit on a European TV satellite last week, is designed to give it a useful life span of at least 25 years. Launched from Kazakhstan’s Baikonur Cosmodrome, the Astra 2A satellite will bring Rupert Murdoch’s 200-channel SkyDigital TV service to British viewers from 1 October. The new propulsion system means it could still be working well after 2020. Every satellite needs some sort of motor to keep it accurately positioned in space. The motor has to be fired every day to compensate for the tugs of solar and lunar gravity, which continually threaten to pull the craft out of its allocated 75-kilometre square of the sky. Until now, these station-keeping motors have used chemical rockets, usually powered by a mixture of monomethyl hydrazine and nitrogen tetroxide. When this fuel runs out, the satellite will wander out of control. This not only makes reception of its signals unreliable, but also brings the risk that it will collide with one of its neighbours in space. So the last few drops of fuel are used to dump the satellite into a “graveyard orbit” where it sits useless but safe before falling to Earth and burning up. Early satellites lasted only a few years. More recent models have a life of 7 to 10 years. Hughes Space and Communications of California started looking for an alternative to chemical thrusters in the early 1960s. First it tried ionising caesium or mercury vapour, but this corroded the jet nozzles. Then in 1984, Hughes engineers found that they could ionise the inert gas xenon to create as much thrust as a chemical jet. Because xenon is inert it does not cause corrosion. This is the system used in Hughes’s xenon ion propulsion system (XIPS), designed to keep the new Astra satellite on station. Years of ground and vacuum chamber testing have finally resulted in a reliable engine that can safely power a spacecraft. In the XIPS motor, xenon gas is fed into a chamber where it is ionised (see Diagram). A series of grid electrodes, like those in an old-fashioned radio valve, accelerate the gas into 3000 parallel streams which leave the chamber through an outlet 13 centimetres in diameter, at a speed of 30 kilometres per second. A neutraliser electrode mounted outside the jet squirts electrons at the positive ions to prevent them flowing back towards the satellite casing, slowing the craft down. The xenon fuel weighs only one-tenth as much as an equivalent amount of chemical fuel. Astra 2A is carrying enough xenon to keep it on station for up to 15 years. In addition, Astra asked Hughes to include a chemical rocket for use as a backup if the xenon thruster fails. If both systems work as intended, Astra 2A’s working life will be almost doubled. “This has a mixed meaning for us,” confesses Hal McDonnell from Hughes, whose job is to sell satellites to broadcasters. But if the new motor performs as planned,